Coding style: consistency

It seems to me that npm 5 starting adding github: as the protocol of
dependencies in package.json which are from GitHub. I personally don't
know which npm version supports these and whether we care about such a
support. Anyway, having some use github: and most not is not consistent.

package.json

@@ -71,7 +71,7 @@
     "redux": "3.7.2",
     "redux-thunk": "2.2.0",
     "styled-components": "1.3.0",
-    "url-polyfill": "github:github/url-polyfill",
+    "url-polyfill": "github/url-polyfill",
     "uuid": "3.1.0",
     "xmldom": "0.1.27"
   },